Ion sensing systems and bio-sensing systems are applied to a wide variety of fields including food preparation and management, environmental measurement and the like. In the ion and bio-sensing areas, there is an increasing demand for ionic and molecular level sensing such as single molecule recognition and single base recognition, and systems and devices with such a sensing ability are needed. For micro-measurement or simultaneous multi-element measurement, there is a need for miniaturization, integration, and on-chip design of such systems and devices.
A typical example of ion sensing device is an ion-sensitive field effect transistor (ISFET) having a silicon nitride film/silicon oxide film/silicon structure. The prior art device uses a separate glass electrode as the reference electrode for pH measurement, having not succeeded in miniaturization and on-chip design. In the present status, a silicon nitride film having a thickness as large as 100 to 200 nanometers (nm) is used as the ion-sensitive film.
In the enzyme, immunity and DNA sensing, on the other hand, sensing based on fluorescence and luminescence using laser scanners has become the main stream. In the recent years, attempts have been made to detect electric current and potential through electrochemical reaction. Also with respect to semiconductor detection, only a few examples pertain to the fabrication of enzyme and immunity sensors combined with the above-mentioned ISFET. The basic detection stance taken in these sensors relies on the quantitative effect to enable detection, typically by increasing the effective surface area of a reactive site or electrode site and by increasing the amount of reactant. Also, the detection using laser scanners and the electrochemical detection suffer from problems since there is a tendency that the response sensitivity (strength, response speed or the like) decreases with a progress of integration and miniaturization.
As discussed above, the prior art techniques are awkward to meet the needs including on-chip design, miniaturization and integration. It is thus believed that an innovative improvement is necessary in order to take advantage of single molecule or ion recognition and detection to the maximum extent. Further, in the ion sensing system and bio-sensing system, there is a particular need for a semiconductor device which is designed for in-solution measurement in the state that a sensor is immersed in liquid so that the detector section is kept in contact with the liquid for a long period of time.
As to the field effect transistor (FET), the inventors reported in Jpn. J. Appl. Phys., Vol. 43, No. 1A/B, 2004, pp. L105-107 (Non-patent Reference 1) a field effect transistor having a gate length of 10 μm and a gate width of 1 mm using a silicon substrate (P—Si(100), 8-12 Ωcm).
This FET has a silicon oxide film formed as a gate dielectric layer as shown in FIG. 19C. Such a FET is prepared by first dry oxidizing a silicon substrate 500, which has been pre-cleaned with 1% HF aqueous solution for about 30 seconds, at a temperature of 1000° C., to form a SiO2 film (field oxide film) 501 of 100 nm thick on the surface of silicon substrate 500 (FIG. 17A). A resist is coated on SiO2 film 501 and patterned (exposed and developed) with UV to form a resist pattern 502 at selected areas (FIG. 17B). Using resist pattern 502 as a mask, SiO2 film 501 is etched with 1% HF aqueous solution to such an extent that a lower layer of SiO2 film 501 is left behind (FIG. 17C). The resist pattern 502 is stripped, forming a channel-gate portion 501a (FIG. 17D).
Next, an aluminum film (thickness 300 nm) was deposited by evaporation (ultimate vacuum 2.0×10−6 Torr, current value 30 mA, deposition rate −5 nm/sec). By a photoresist process, the aluminum film is formed into a predetermined aluminum film pattern 503 (FIG. 18A), which functions as a mask for subsequent ion implantation. By ion implantation (P-dope, 40 kV, 1.0×1015 ions/cm2) using aluminum film pattern 503 as a mask, N-channels 504, 504 are formed in predetermined areas of an upper layer of silicon substrate 500. The aluminum film pattern 503 is stripped off (by immersing in 50% phosphoric acid at 80° C. for 5 minutes).
After aluminum film pattern 503 is stripped off (FIG. 18B), the surface of SiO2 film 501 is annealed in a N2 atmosphere (900° C., 5 min) for activation. A resist is then coated on SiO2 film 501 and patterned (exposed and developed) with UV to form a resist pattern 505 which cover areas of SiO2 film 501 other than the areas in register with N-channels 504, 504 (FIG. 18C). The SiO2 film 501 on N-channels 504, 504 is etched (1% HF aqueous solution) using resist pattern 505 as a mask, and resist pattern 505 is removed, forming contact holes 504a, 504a (FIG. 18D).
Next, an electrode metallization 506 is formed by evaporation (EB evaporation, ultimate vacuum 2.0×10−8 Torr). Specifically, a titanium film (thickness 20 nm, vacuum during deposition 4.0×10−8 Torr, current value 70 mA, deposition rate 0.13 nm/sec) and a platinum film (thickness 120 nm, vacuum during deposition 8.0×10−8 Torr, current value 220 mA, deposition rate 0.067 nm/sec) are deposited to form electrode metallization 506 (FIG. 19A), which is annealed in a nitrogen atmosphere (800° C., 10 min) to produce TiSi2 at the junction between the Ti film of electrode metallization 506 and N-channels 504, 504, forming contacts.
Then a protective oxide film 507 (thickness 200 nm) is formed on electrode metallization 506 by plasma enhanced CVD (PECVD, 200 W, 400° C., 0.39 Torr, tetraethoxysilane (TEOS) 6 sccm, O2 100 sccm) (FIG. 19B). Structural recovery treatment is carried out on the CVD oxide film by annealing in an oxygen atmosphere (800° C., 10 min). Gate/electrode contact holes 508, 508 are perforated by reactive ion etching (RIE) with CHF3 gas, yielding a field effect transistor as shown in FIG. 19C.
When such a FET is used as a semiconductor sensing device, it is modified on the gate dielectric layer with an organic monomolecular film or the like. Since a sensor of the type shown in FIG. 19C has the structure that the gate dielectric layer composed of the silicon oxide film is exposed, entry of moisture, ions and the like can impair the transistor characteristics. This sensor is unsuited for long-term measurement with the detector section kept in contact with liquid.
Further, in the ion sensing system and bio-sensing system, for example, there is a particular need for a semiconductor device which is designed for in-solution measurement in the state that not only the sensor section, but also a meter section for measuring the electric signal detected by the sensor section are kept in contact with the liquid for a long period of time.
Especially in the medical field where an ever increasing demand for semiconductor sensors is expected in the future, a possibility of cleaning the sensor section for reuse is low from the safe hygienic aspect. Nevertheless, based on the presumption that the electronic part is brought in contact with liquid such as aqueous solution, prior art semiconductor sensing devices are assembled integral from the water- and liquid-proof standpoint so that the sensor section and the meter section are not readily detachable. The operation of exchanging the sensor section is so complex that disposable instruments are unfeasible.
Further, in order that the sensor section and the meter section be readily detachable, the water- and liquid-tightness at the joint between the sensor section and the meter section is also important in a semiconductor sensing system for which water- and liquid-proofness is required. In the case of disposable instruments in which the sensor section and the meter section are readily detachable so that the sensor section is replaced on every use, tight closure is necessary because entry of moisture or the like through the joint can cause failure of the instrument. For a semiconductor sensor which is susceptible to failure by external forces, a tight closure method matching with the strength thereof is required.
Patent Reference 1:                JP-A 2004-4007        
Non-Patent Reference 1:                Daisuke Niwa et al., Jpn. J. Appl. Phys.,        Vol. 43, No. 1A/B, 2004, pp. L105-107        